The present invention relates generally to a line monitoring system employed in a lightwave communication system, and more particularly to a line monitoring system employing digital command and response signaling.
Commercial lightwave systems use optical fibers to carry large amounts of multiplexed digital data over long distances from a transmit terminal to a receive terminal. The maximum distance that the data can be transmitted in the fiber without amplification or regeneration is limited by the loss and dispersion associated with the optical fiber. To transmit optical signals over long distances, the lightwave systems may include a number of repeaters periodically located along the fiber route from the transmit terminal to the receive terminal. Each repeater boosts the weak received signal to compensate for the transmission losses which occurred from the last repeater. Prior to the widespread availability of efficient optical amplifiers, many systems converted the optical signals into electrical signals for amplification by conventional electrical amplifiers. The amplified electrical signals were then reconverted to the optical domain, for further distribution along the optical communication path. The advent of reliable and low cost optical amplifiers has obviated the need to convert signals into the electrical domain for amplification.
Optical amplifiers, such as rare earth doped optical fiber amplifiers, require a source of pump energy. In a rare earth doped optical fiber amplifier, for example, a dedicated pump laser is coupled to the doped fiber for exciting the active medium (rare earth element) within the amplifier. At the same time, a communication signal is passed through the doped fiber. The doped fiber exhibits gain at the wavelength of the communication signal, providing the desired amplification. If the doped optical fiber is doped with erbium, for example, pump energy may be provided at a wavelength of 1485 nm or 980 nm, which coincide with the absorption peaks of erbium.
Optical communication systems often employ a line monitoring system (LMS) to monitor the performance of the repeaters and to control repeater functions. In particular, useful information that may be monitored includes degradations in pump power, loss in the amplifier, and loss in the transmission span. Repeater functions that can be controlled include power switching and laser switching, for example. The LMS may used when the communication system is in-service, out-of-service, or under degraded system operating conditions. One type of LMS that can be used is a digital command and response arrangement in which a terminal transmits a command signal over the transmission line. The command signal is addressed to a particular repeater or repeaters and specifies either the status information that is requested or the command and control function that is to be executed. The repeater generates a response signal that is returned to the requesting terminal over the opposite-going transmission path. The response signal includes the status information that was requested by the terminal. The response signal is transmitted in the form of AM modulation imposed on the optical amplifier gain, which requires that the power of the laser pump that pumps the amplifier undergo a corresponding modulation. Thus, to generate the response signal the average output power of the laser pump must be increased above its nominal value by a selected amount corresponding to the depth of modulation. The greater the modulation depth, the greater the SNR of the response signal arriving at the terminal. Unfortunately, it is difficult to increase the output power of a laser operating at 980 nm above its average output power without significantly impairing its reliability. As a result, the modulation depth of the response signal is limited.
Accordingly, it would be desirable to provide a command and response line monitoring system in which the SNR of the response signal can be increased for any purpose, including the previously described situation where it is desirable to increase the SNR of the response signal without requiring an increase in the average optical output power of the laser pump.
The present invention provides a method for transmitting an optical response signal from a repeater to a terminal requesting status information pertaining to the repeater. In accordance with the method, in response to a command signal, a bias current applied to a laser pump within the repeater is modulated such that the laser pump generates modulated optical output power. The modulation reflects the status information that is requested. An average value of the bias current applied to the laser pump is reduced while the bias current is being modulated. The modulated optical output power is applied to a doped optical fiber located within the repeater to amplitude modulate a communication signal traversing the doped optical fiber. A portion of the amplitude modulated communication signal is directed to the requesting terminal. The portion of the amplitude modulated communication signal may be received to detect a response signal embodied in the amplitude modulation.
Since the average bia current applied to the laser pump is decreased while undergoing modulation, the modulation depth may be increased without a corresponding increase in the average optical output power of the laser pump. Therefore, the signal-to-noise ratio of the response signal is increased since the modulation depth is increased.